Statistics
Scientific paper
Jan 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992phdt........45g&link_type=abstract
Thesis (PH.D.)--MASSACHUSETTS INSTITUTE OF TECHNOLOGY, 1992.Source: Dissertation Abstracts International, Volume: 53-04, Secti
Statistics
35
Galaxies, Cold Dark Matter Model
Scientific paper
The goal of this work is to understand how galaxies may have formed and clustered as a result of gravitational instability amplifying primordial fluctuations generated in the Big Bang. We present new results concerning the number of halos and the spatial and velocity statistics of halos that form in the Omega = 1 Cold Dark Matter (CDM) model. We report results from CDM simulations using up to 256^3 particles with the particle -mesh (PM) algorithm and using up to 144^3 particles with the adaptive particle-particle- -particle-mesh (P^3M) algorithm. Our most important innovation here has been the implementation of an algorithm that identifies halos by finding particles that are gravitationally bound to local maxima of the smoothed, evolved nonlinear density field. We refer to a given amplitude as sigma _8, i.e. the linear, r.m.s. mass fluctuations in spheres of radii 8h^{-1} Mpc. The number of halos characterized by their circular velocities is in reasonable agreement with the observations over the range 150 _sp{~ }< V_{rm circ} _sp{~}< 350 km s^{-1}. However, there are too many halos with V_{rm circ} _sp{~}< 125km s^{-1} , and there are too many massive halos with radial velocity dispersions exceeding 350 km s^ {-1} for sigma_8 _sp{~}> 0.4. The halos have more substructure than predicted by the Press-Schechter theory. We compared our simulations with observed clusters selected from the CfA-2 redshift survey. Our simulations fail to produce the observed numbers of clusters and there are too many field galaxies. If we break up the massive halos to make clusters of galaxies, there are too many rich clusters. There is no value of sigma _8 when we can simultaneously match number of halos and their spatial and velocity statistics to the observations with or without the break-up of the massive halos. The best hope for CDM is sigma_8 _sp{~}< 0.5. We compare direct measures of clustering statistics from the halos with the method of Davis and Peebles applied to our simulations. For values of sigma _8 that give reasonable agreement with the observations, sigma_8 _sp {~}< 0.5, both direct estimates and the Davis and Peebles method are in good agreement. (Copies available exclusively from MIT Libraries, Rm. 14-0551, Cambridge, MA 02139-4307. Ph. 617 -253-5668; Fax 617-253-1690.) (Abstract shortened with permission of school.).
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